From Windows & .NET Magazine.
XP SP2 lets you run two sessions (one local console and one remote desktop) concurrently. You must use a different user for each session. To enable concurrent sessions, perform the following steps:
Start a registry editor (e.g., regedit.exe).
Navigate to the HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlTerminal ServerLicensing Core registry subkey.
From the Edit menu, select New, DWORD Value.
Enter the name “EnableConcurrentSessions,” then press Enter.
Double-click the new value, then set it to 1.